The present invention relates to a vehicle steering control system for controlling at least a rear wheel steer angle of a vehicle.
Japanese Patent Provisional Publication No. 60-229873 shows a rear wheel steer angle control system which steers rear wheels of a vehicle so that an actual rear wheel steer angle is given by; EQU .delta..sub.r =K.times..theta.-.tau..times..theta.
where .delta..sub.r is the rear wheel steer angle, .theta. is a steering angle of the vehicle, .theta. is a steering angular speed, K is a proportional constant (or coefficient), and .tau.(tau) is a derivate constant (coefficient). The derivative constant is positive.
Similar rear wheel steer angle control systems are disclosed also in many commonly-assigned, copending U.S. patent applications. Examples are Ser. No. 07/195,085 (, filed by F. SUGASAWA, on May 17, 1988, for "VEHICLE STEER ANGLE CONTROL SYSTEM"), Ser. No. 07/195,078 (, filed by K. MORI, on May 17, 1988, for "SYSTEM FOR CONTROLLING STEER ANGLE OF REAR WHEELS OF FOUR WHEEL STEERABLE MOTOR VEHICLE"), Ser. No. 07/275,061 (, filed by K. KAWAGOE, on Nov. 22, 1988, for "VEHICLE STEERING CONTROL SYSTEM WITH DERIVATIVE GAIN ADJUSTING CAPABILITY"), Ser. No. 07/277,745 (, filed by K. MORI et al., on Nov. 30, 1988, for "REAR WHEEL STEER ANGLE CONTROL SYSTEM FOR VEHICLE"), Ser. No. 07/277,744 (, filed by K. MORI et al., on Nov. 30, 1988, for "FOUR-WHEEL STEER CONTROL SYSTEM"), Ser. No. 07/269,698 (, filed by F. SUGASAWA et al. on Nov. 10, 1988, for "METHOD FOR STEERING VEHICLE"), Ser. No. 07/284,414 (, filed by T. EGUCHI et al., on Dec. 14, 1988, for "VEHICLE REAR WHEEL STEER ANGLE CONTROL SYSTEM") and Ser. No. 07/305,023 (filed by F. SUGASAWA, on Feb. 2, 1989, for "FAIL-SAFE STEER ANGLE CONTROL SYSTEM FOR VEHICLE").
The proportional coefficient K and the derivative coefficient tau assume different values in accordance with the make of the vehicle and the vehicle speed. The proportional coefficient K is generally positive. For example, K equals 0.38, and minus tau equals -0.049 when the mass M of the vehicle equals 160 kgS.sup.2 /m, the yawing moment of inertia I of the vehicle equals 252 kgmS.sup.2, the wheelbase 1 equals 2.48 m, and the vehicle speed V equals 120 km/h. However, K =0.33 and minus tau =-0.056 for the same vehicle speed, V =120 km/h if M =125 kgS.sup.2 /m, I =227 kgS.sup.2 and 1 =2.48 m.
When the steering wheel is turned speedily, the control system of this type steers the rear wheels so that the rear wheel steer angle remains for a limited time in an opposite phase side in which the rear wheels are steered in the opposite direction to the steering direction of the front wheels, and then changes to a same phase side in which the rear wheels are steered in the same direction as the front wheels (so-called phase inversion control).
However, this control system takes no account of the vehicle lateral acceration in determining a value of the derivative coefficient, so that it cannot improve the vehicle steering response of yawing motion satisfactorily.
In general, it is desired to improve the yawing characteristic of a vehicle in the low vehicle speed range. In the high vehicle speed range, it is desired to improve the stability of the vehicle. In the control system using the above-mentioned equation, the vehicle yawing characteristic is mainly determined by the derivative term .tau..theta. which is the product obtained by multiplying the steering speed .theta. by tau. This derivative term acts to increase the opposite phase steering amount through which the rear wheels are steered in the direction opposite to the steering direction of the front wheels, or to reduce the same phase steering amount through which the rear wheels are steered in the same direction as the front wheel steering direction. The derivative term .tau..theta. increases with increase in the steering speed .theta., provided that the derivative coefficient tau is fixed. On the other hand, the derivative term increases with increase in the derivative coefficient tau, provided that the steering speed .theta. is fixed. As the derivative term is increased, the opposite phase steering amount is increased, and accordingly the vehicle yawing characteristic is improved.
The yawing characteristic in the range of low lateral accelerations can be increased by increasing the derivative coefficient tau and thereby increasing the opposite phase steering amount. However, in this case, there appears a momentary lateral acceleration in the outward direction away from the center of the turning arc, which arouses unnatural feeling in the driver. Therefore, there is a limit to the increase of the derivative coefficient tau.
In a turn of a high lateral acceleration, provided that the steering conditions (the steering angle .theta. and the steering speed .theta. are the same), it is necessary to make the opposite phase steering amount higher than the level required in the low lateral G range, in order to obtain the yawing characteristic of the same rank. FIG. 4 shows a characteristic of a tire cornering force. When the vehicle is in a turn of a low lateral G or in a straight ahead operation (in a range A' in FIG. 4), the side slip angle is still small, and the rate of increase of the cornering force with respect to the side slip angle (the slope of a tangent line a shown in FIG. 4) is high. When the lateral G is medium (a range B'), the side slip angle becomes greater, but the rate of increase (the slope of a tangent line b) becomes lower than that of the low G turn. When the lateral G is high (a range C'), the side slip angle reaches its maximum and then decreases, and the rate of increase of the cornering force (the slope of a tangent line c) is further decreased. During a turn of a high lateral G, therefore, the vehicle slip angle is large, and the cornering force produced per unit steer angle is low. Therefore, in the high lateral G range, it is desired to improve the yawing characteristic by increasing the opposite phase steering amount beyond the level of the low lateral G range. However, the control system of the conventional example cannot satisfy both of the demand for improving the yawing characteristic in the low lateral G range, and the demand for improving the yawing characteristic in the high lateral G range.